Domestic Waste Water Treatment

1. What is Domestic Wastewater and why treat it ?
Wastewater derived from human activities in households such as bath, laundry, dish washing, garbage disposal, toilets etc is called as Domestic Wastewater which usually contains relatively small amounts of contaminants but even small amount of pollutants can make a big impact on environment. Hence, a properly installed and maintained residential sewage treatment system for treating and disposing of household wastewater will minimize the impact on ground water and surface water.

Domestic waste water treatment plays an important role in nowadays. Domestic waste treatment ensures that all household sewage is properly treated to make it safe, clean and suitable for releasing back into the environment, lakes, or streams. Home sewage systems are designed to treat all of the liquid waste generated from a residence. Possible contaminants in household wastewater include disease-causing bacteria, infectious viruses, household chemicals, and excess nutrients such as nitrate.

Domestic waste consists of two main fluxes: one is grey water which is from kitchen sinks, wash basins, laundry washing, showers, baths etc., and second one is black water which is from toilets and urinals.
Household sewage treatment plant breaks down domestic wastes via three major stages.

1. Primary Stage:
a. This is the first stage of sewage/wastewater treatment that removes about 40-60% of the suspended solids.
b. It involves screening to remove large objects such as sticks, stones etc which can cause damage to tank inlets.
c. It employs grit chamber which slows down the flow of wastewater to allow grit to fall out naturally to the bottom of the     tank where it can be removed.
d. Primary clarifier or settling/sedimentation tank in this stage removes sinking and floating contaminants.
e. The partially treated wastewater from the primary tanks then flows to the secondary treatment system.

2. Secondary Stage:
a. This is the stage where the biological (aerobic/anaerobic) treatment of waste water from the primary stage begins and it     removes up to 90% of organic matter.
b. It uses activated sludge process which use dissolved oxygen to promote growth of biological floc that substantially removes     organic matter.
c. Bacteria-containing “activated sludge” is continually re-circulated back to the aeration tank to increase the rate of organic     decomposition.
d. Bacteria attack the dissolved and finely divided suspended solids which are not removed by primary sedimentation.
e. The water is then taken to settling tanks where the sludge again settles, leaving the water 90 to 95 % free of pollutants.

3. Tertiary Stage:
a. When the effluent from secondary treatment is unacceptable, a third level of treatment called tertiary or advanced     treatment, can be employed.
b. Its purpose is to provide final treatment stage to raise the effluent quality to the desired level.
c. When the wastewater reaches tertiary stage, it still contain suspended matter and fine particles and are removed in this     stage.
d. The water at this stage is almost free from harmful substances and chemicals and which can be reused, recycled or released     back into the environment.
e. This stage is also called as disinfection stage and UV is an ideal disinfectant for wastewater since it does not alter the water     quality.

Also find